Interbody spinal implant havng a roughened surface topography

ABSTRACT

An interbody spinal implant including a body having a top surface, a bottom surface, opposing lateral sides, opposing anterior and posterior portions, a substantially hollow center, and a single vertical aperture. The single vertical aperture extends from the top surface to the bottom surface, has a size and shape predetermined to maximize the surface area of the top surface and the bottom surface available proximate the anterior and posterior portions while maximizing both radiographic visualization and access to the substantially hollow center, and defines a transverse rim. The body may be non-metallic and may form one component of a composite implant; the other component is a metal plate disposed on at least one of the top and bottom surfaces of the body.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/572,077 which was filed on Aug. 10, 2012, which is a continuation ofU.S. patent application Ser. No. 12/151,198, filed May 5, 2008, andissued as U.S. Pat. No. 8,262,737 on Sep. 11, 2012, and is a ofcontinuation-in-part of U.S. patent application Ser. No. 11/123,359,filed on May 6, 2005 and issued as U.S. Pat. No. 7,662,186 on Feb. 16,2010, the contents of each are incorporated by reference into thisdocument, in their entirety and for all purposes.

TECHNICAL FIELD

The present invention relates generally to interbody spinal implants andmethods of using such implants and, more particularly, to an implanthaving one or more openings of predetermined sizes and shapes to achievedesign trade offs depending upon a particular application.

BACKGROUND OF THE INVENTION

In the simplest terms, the spine is a column made of vertebrae anddiscs. The vertebrae provide the support and structure of the spinewhile the spinal discs, located between the vertebrae, act as cushionsor “shock absorbers.” These discs also contribute to the flexibility andmotion of the spinal column. Over time, the discs may become diseased orinfected, may develop deformities such as tears or cracks, or may simplylose structural integrity (e.g., the discs may bulge or flatten).Impaired discs can affect the anatomical functions of the vertebrae, dueto the resultant lack of proper biomechanical support, and are oftenassociated with chronic back pain.

Several surgical techniques have been developed to address spinaldefects, such as disc degeneration and deformity. Spinal fusion hasbecome a recognized surgical procedure for mitigating back pain byrestoring biomechanical and anatomical integrity to the spine. Spinalfusion techniques involve the removal, or partial removal, of at leastone intervertebral disc and preparation of the disc space for receivingan implant by shaping the exposed vertebral endplates. An implant isthen inserted between the opposing endplates.

Spinal fusion procedures can be achieved using a posterior or ananterior approach. Anterior interbody fusion procedures generally havethe advantages of reduced operative times and reduced blood loss.Further, anterior procedures do not interfere with the posterioranatomic structure of the lumbar spine. Anterior procedures alsominimize scarring within the spinal canal while still achieving improvedfusion rates, which is advantageous from a structural and biomechanicalperspective. These generally preferred anterior procedures areparticularly advantageous in providing improved access to the discspace, and thus correspondingly better endplate preparation.

Several interbody implant systems have been introduced to facilitateinterbody fusion. Traditional threaded implants involve at least twocylindrical bodies, each typically packed with bone graft material,surgically placed on opposite sides of the mid-sagittal plane throughpre-tapped holes within the intervertebral disc space. This location isnot the preferable seating position for an implant system, however,because only a relatively small portion of the vertebral endplate iscontacted by these cylindrical implants. Accordingly, these implantbodies will likely contact the softer cancellous bone rather than thestronger cortical bone, or apophyseal rim, of the vertebral endplate.The seating of these threaded cylindrical implants may also compromisebiomechanical integrity by reducing the area in which to distributemechanical forces, thus increasing the apparent stress experienced byboth the implant and vertebrae. Still further, a substantial risk ofimplant subsidence (defined as sinking or settling) into the softercancellous bone of the vertebral body may arise from such improperseating.

In contrast, open ring-shaped cage implant systems are generally shapedto mimic the anatomical contour of the vertebral body. Traditionalring-shaped cages are generally comprised of allograft bone material,however, harvested from the human femur. Such allograft bone materialrestricts the usable size and shape of the resultant implant. Forexample, many of these femoral ring-shaped cages generally have amedial-lateral width of less than 25 mm. Therefore, these cages may notbe of a sufficient size to contact the strong cortical bone, orapophyseal rim, of the vertebral endplate. These size-limited implantsystems may also poorly accommodate related instrumentation such asdrivers, reamers, distractors, and the like. For example, these implantsystems may lack sufficient structural integrity to withstand repeatedimpact and may fracture during implantation. Still further, othertraditional non-allograft ring-shaped cage systems may be size-limiteddue to varied and complex supplemental implant instrumentation which mayobstruct the disc space while requiring greater exposure of theoperating space. These supplemental implant instrumentation systems alsogenerally increase the instrument load upon the surgeon.

The surgical procedure corresponding to an implant system shouldpreserve as much vertebral endplate bone surface as possible byminimizing the amount of bone removed. This vertebral endplate bonesurface, or subchondral bone, is generally much stronger than theunderlying cancellous bone. Preservation of the endplate bone stockensures biomechanical integrity of the endplates and minimizes the riskof implant subsidence. Thus, proper interbody implant design shouldprovide for optimal seating of the implant while utilizing the maximumamount of available supporting vertebral bone stock.

Traditional interbody spinal implants generally do not seat properly onthe preferred structural bone located near the apophyseal rim of thevertebral body, which is primarily composed of preferred densesubchondral bone. Accordingly, there is a need in the art for interbodyspinal implants which better utilize the structurally supportive bone ofthe apophyseal rim.

In summary, at least ten, separate challenges can be identified asinherent in traditional anterior spinal fusion devices. Such challengesinclude: (1) end-plate preparation; (2) implant difficulty; (3)materials of construction; (4) implant expulsion; (5) implantsubsidence; (6) insufficient room for bone graft; (7) stress shielding;(8) lack of implant incorporation with vertebral bone; (9) limitationson radiographic visualization; and (10) cost of manufacture andinventory. Each of these challenges is addressed in turn.

1. End-Plate Preparation

There are three traditional end-plate preparation methods. The first isaggressive end-plate removal with box-chisel types of tools to create anice match of end-plate geometry with implant geometry. In the processof aggressive end-plate removal, however, the end-plates are typicallydestroyed. Such destruction means that the load-bearing implant ispressed against soft cancellous bone and the implant tends to subside.

The second traditional end-plate preparation method preserves theend-plates by just removing cartilage with curettes. The end-plates areconcave; hence, if a flat implant is used, the implant is not verystable. Even if a convex implant is used, it is very difficult to matchthe implant geometry with the end-plate geometry, as the end-plategeometry varies from patient-to-patient and on the extent of disease.

The third traditional end-plate preparation method uses threaded fusioncages. The cages are implanted by reaming out corresponding threads inthe end-plates. This method also violates the structure.

2. Implant Difficulty

Traditional anterior spinal fusion devices can also be difficult toimplant. Some traditional implants with teeth have sharp edges. Theseedges can bind to the surrounding soft tissue during implantation,creating surgical challenges.

Typically, secondary instrumentation is used to keep the disc spacedistracted during implantation. The use of such instrumentation meansthat the exposure needs to be large enough to accommodate theinstrumentation. If there is a restriction on the exposure size, thenthe maximum size of the implant available for use is correspondinglylimited. The need for secondary instrumentation for distraction duringimplantation also adds an additional step or two in surgery. Stillfurther, secondary instrumentation may sometimes over-distract theannulus, reducing the ability of the annulus to compress a relativelyundersized implant. The compression provided by the annulus on theimplant is important to maintain the initial stability of the implant.

For anterior spinal surgery, there are traditionally three trajectoriesof implants: anterior, antero-lateral, and lateral. Each approach hasits advantages and drawbacks. Sometimes the choice of the approach isdictated by surgeon preference, and sometimes it is dictated by patientanatomy and biomechanics. A typical traditional implant has designfeatures to accommodate only one or two of these approaches in a singleimplant, restricting intra-operative flexibility.

3. Materials of Construction

Other challenges raised by traditional devices find their source in theconventional materials of construction. Typical devices are made of PEEKor cadaver bone. Materials such as PEEK or cadaver bone do not have thestructural strength to withstand impact loads required duringimplantation and may fracture during implantation.

PEEK is an abbreviation for polyetherether-ketone, a high-performanceengineering thermoplastic with excellent chemical and fatigue resistanceplus thermal stability. With a maximum continuous working temperature of480° F., PEEK offers superior mechanical properties. Superior chemicalresistance has allowed PEEK to work effectively as a metal replacementin harsh environments. PEEK grades offer chemical and water resistancesimilar to PPS (polyphenylene sulfide), but can operate at highertemperatures. PEEK materials are inert to all common solvents and resista wide range of organic and inorganic liquids. Thus, for hostileenvironments, PEEK is a high-strength alternative to fluoropolymers.

The use of cadaver bone has several drawbacks. The shapes and sizes ofthe implants are restricted by the bone from which the implant ismachined. Cadaver bone carries with it the risk of disease transmissionand raises shelf-life and storage issues. In addition, there is alimited supply of donor bone and, even when available, cadaver boneinherently offers inconsistent properties due to its variability.Finally, as mentioned above, cadaver bone has insufficient mechanicalstrength for clinical application.

4. Implant Expulsion

Traditional implants can migrate and expel out of the disc space,following the path through which the implant was inserted. Typicalimplants are either “threaded” into place, or have “teeth” which aredesigned to prevent expulsion. Both options can create localized stressrisers in the end-plates, increasing the chances of subsidence. Thechallenge of preventing implant expulsion is especially acute for PEEKimplants, because the material texture of PEEK is very smooth and“slippery.”

5. Implant Subsidence

Subsidence of the implant is a complex issue and has been attributed tomany factors. Some of these factors include aggressive removal of theend-plate; an implant stiffness significantly greater than the vertebralbone; smaller sized implants which tend to seat in the center of thedisc space, against the weakest region of the end-plates; and implantswith sharp edges which can cause localized stress fractures in theend-plates at the point of contact. The most common solution to theproblem of subsidence is to choose a less stiff implant material. Thisis why PEEK and cadaver bone have become the most common materials forspinal fusion implants. PEEK is softer than cortical bone, but harderthan cancellous bone.

6. Insufficient Room for Bone Graft

Cadaver bone implants are restricted in their size by the bone fromwhich they are machined. Their wall thickness also has to be great tocreate sufficient structural integrity for their desired clinicalapplication. These design restrictions do not leave much room forfilling the bone graft material into cortical bone implants. Theexposure-driven limitations on implant size narrow the room left insidethe implant geometry for bone grafting even for metal implants. Suchroom is further reduced in the case of PEEK implants because their wallthickness needs to be greater as compared to metal implants due tostructural strength needs.

7. Stress Shielding

For fusion to occur, the bone graft packed inside the implant needs tobe loaded mechanically. Typically, however, the stiffness of the implantmaterial is much greater than the adjacent vertebral bone and takes up amajority of the mechanical loads, “shielding” the bone graft materialfrom becoming mechanically loaded. The most common solution is to choosea less stiff implant material. Again, this is why PEEK and cadaver bonehave become the most common materials for spinal fusion implants. Asnoted above, although harder than cancellous bone, PEEK is softer thancortical bone.

8. Lack of Implant Incorporation with Vertebral Bone

In most cases, the typical fusion implant is not able to incorporatewith the vertebral bone, even years after implantation. Such inabilitypersists despite the use of a variety of different materials toconstruct the implants. There is a perception that cadaver bone isresorbable and will be replaced by new bone once it resorbs. Hedrocel isa composite material composed of carbon and tantalum, an inert metal,that has been used as a material for spinal fusion implants. Hedrocel isdesigned to allow bone in-growth into the implant. In contrast, PEEK hasbeen reported to become surrounded by fibrous tissue which precludes itfrom incorporating with surrounding bone. There have also been reportsof the development of new bio-active materials which can incorporateinto bone. The application of such bio-active materials has beenlimited, however, for several reasons, including biocompatibility,structural strength, and lack of regulatory approval.

9. Limitations on Radiographic Visualization

For implants made out of metal, the metal prevents adequate radiographicvisualization of the bone graft. Hence it is difficult to assess fusion,if it is to take place. PEEK is radiolucent. Traditional implants madeof PEEK need to have radiographic markers embedded into the implants sothat implant position can be tracked on an X-ray. Cadaver bone has someradiopacity and does not interfere with radiographic assessment as muchas metal implants.

10. Cost of Manufacture and Inventory

The requirements of spinal surgery dictate that manufacturers provideimplants of various foot-prints, and several heights in each foot-print.This requirement means that the manufacturer needs to carry asignificant amount of inventory of implants. Because there are so manydifferent sizes of implants, there are setup costs involved in themanufacture of each different size. The result is increased implantcosts, which the manufacturers pass along to the end users by charginghigh prices for spinal fusion implants.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to interbody spinal implants and tomethods of using such implants. The implants can be inserted, usingmethods of the present invention, from a variety of vantages, includinganterior, antero-lateral, and lateral implantation. Certain embodimentsof the present invention provide an anatomically shaped spinal implantfor improved seating in the disc space, particularly in themedial-lateral aspect of the disc space, and improved utilization of thevertebral apophyseal rim. Certain embodiments of the present inventionfurther have a highly radiused posterior portion and sides which allowfor ease of implantation. Thus, the posterior portion may have agenerally blunt nosed profile. Certain embodiments also allow forimproved visualization of the disc space during surgical procedureswhile minimizing exposure of the operating space. Certain aspects of theinvention reduce the need for additional instrumentation—such aschisels, reamers, or other tools—to prepare the vertebral endplate, thusminimizing the instrument load upon the surgeon.

Certain embodiments of the interbody implant are substantially hollowand have a generally oval-shaped transverse cross-sectional area.Substantially hollow, as used in this document, means at least about 33%of the interior volume of the interbody spinal implant is vacant.Further embodiments of the present invention include a body having a topsurface, a bottom surface, opposing lateral sides, and opposing anteriorand posterior portions. The implant includes at least one aperture thatextends the entire height of the body. Thus, the aperture extends fromthe top surface to the bottom surface. The implant may further includeat least one aperture that extends the entire transverse length of theimplant body.

Still further, the substantially hollow portion may be filled withcancellous autograft bone, allograft bone, demineralized bone matrix(DBM), porous synthetic bone graft substitute, bone morphogenic protein(BMP), or combinations of those materials. The implant further includesa roughened surface topography on at least a portion of its top surface,its bottom surface, or both surfaces. The anterior portion, or trailingedge, of the implant is preferably generally greater in height than theopposing posterior portion, or leading edge. In other words, thetrailing edge is taller than the leading edge. The posterior portion andlateral sides may also be generally smooth and highly radiused, thusallowing for easier implantation into the disc space. Thus, theposterior portion may have a blunt nosed profile. The anterior portionof the implant may preferably be configured to engage a delivery device,a driver, or other surgical tools. The anterior portion may also besubstantially flat.

According to certain embodiments, the present invention provides aninterbody spinal implant including a body having a top surface, a bottomsurface, opposing lateral sides, opposing anterior and posteriorportions, a substantially hollow center, and a single vertical aperture.The single vertical aperture extends from the top surface to the bottomsurface, has a size and shape predetermined to maximize the surface areaof the top surface and the bottom surface available proximate theanterior and posterior portions while maximizing both radiographicvisualization and access to the substantially hollow center, and definesa transverse rim. The body may be non-metallic and may form onecomponent of a composite implant; the other component is a metal platedisposed on at least one of the top and bottom surfaces of the body.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 shows a perspective view of a first embodiment of the interbodyspinal implant having a generally oval shape and roughened surfacetopography on the top surface;

FIG. 2 depicts a top view of the first embodiment of the interbodyspinal implant;

FIG. 3 depicts an anterior view of the first embodiment of the interbodyspinal implant;

FIG. 4 depicts a posterior view of the first embodiment of the interbodyspinal implant;

FIG. 5A depicts a first post-operative radiograph showing visualizationof an embodiment of the interbody spinal implant;

FIG. 5B depicts a second post-operative radiograph showing visualizationof an embodiment of the interbody spinal implant;

FIG. 5C depicts a third post-operative radiograph showing visualizationof an embodiment of the interbody spinal implant;

FIG. 6 shows an exemplary surgical tool (implant holder) to be used withcertain embodiments of the interbody spinal implant;

FIG. 7 shows an exemplary distractor used during certain methods ofimplantation;

FIG. 8 shows an exemplary rasp used during certain methods ofimplantation;

FIG. 9 shows a perspective view from the front of another embodiment ofthe interbody spinal implant according to the present invention;

FIG. 10 shows a perspective view from the rear of the embodiment of theinterbody spinal implant illustrated in FIG. 9;

FIG. 11 is a top view of the interbody spinal implant illustrated inFIGS. 9 and 10;

FIG. 12 shows a perspective view from the rear, like FIG. 10, of theinterbody spinal implant illustrated in FIGS. 9-11 highlighting analternative transverse aperture;

FIG. 13 shows a perspective view from the front of yet anotherembodiment of the interbody spinal implant according to the presentinvention;

FIG. 14 is a top view of the interbody spinal implant illustrated inFIG. 13;

FIG. 15 shows a perspective view from the rear of the embodiment of theinterbody spinal implant illustrated in FIG. 13 highlighting analternative transverse aperture;

FIG. 16 shows a perspective view from the side of one component of acomposite embodiment of the interbody spinal implant;

FIG. 17 is a top view of the composite embodiment of the interbodyspinal implant illustrated in FIG. 16 with the components attached;

FIG. 18 shows an exemplary mechanism by which the two components of thecomposite embodiment of the interbody spinal implant illustrated inFIGS. 16 and 17 may be attached;

FIG. 19 shows a perspective view of a final embodiment of the interbodyspinal implant having a generally oval shape and being especially welladapted for use in a cervical spine surgical procedure; and

FIG. 20 shows a perspective view of the final implant having a generallybox shape.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the present invention may be especially suitedfor placement between adjacent human vertebral bodies. The implants ofthe present invention may be used in procedures such as Anterior LumbarInterbody Fusion (ALIF), Posterior Lumbar Interbody Fusion (PLIF),Transforaminal Lumbar Interbody Fusion (TLIF), and cervical fusion.Certain embodiments do not extend beyond the outer dimensions of thevertebral bodies.

The ability to achieve spinal fusion is directly related to theavailable vascular contact area over which fusion is desired, thequality and quantity of the fusion mass, and the stability of theinterbody spinal implant. Interbody spinal implants, as now taught,allow for improved seating over the apophyseal rim of the vertebralbody. Still further, interbody spinal implants, as now taught, betterutilize this vital surface area over which fusion may occur and maybetter bear the considerable biomechanical loads presented through thespinal column with minimal interference with other anatomical orneurological spinal structures. Even further, interbody spinal implants,according to certain aspects of the present invention, allow forimproved visualization of implant seating and fusion assessment.Interbody spinal implants, as now taught, may also facilitateosteointegration with the surrounding living bone.

Anterior interbody spinal implants in accordance with certain aspects ofthe present invention can be preferably made of a durable material suchas stainless steel, stainless steel alloy, titanium, or titanium alloy,but can also be made of other durable materials such as, but not limitedto, polymeric, ceramic, and composite materials. For example, certainembodiments of the present invention may be comprised of abiocompatible, polymeric matrix reinforced with bioactive fillers,fibers, or both. Certain embodiments of the present invention may becomprised of urethane dimethacrylate (DUDMA)/tri-ethylene glycoldimethacrylate (TEDGMA) blended resin and a plurality of fillers andfibers including bioactive fillers and E-glass fibers. Durable materialsmay also consist of any number of pure metals, metal alloys, or both.Titanium and its alloys are generally preferred for certain embodimentsof the present invention due to their acceptable, and desirable,strength and biocompatibility. In this manner, certain embodiments ofthe present interbody spinal implant may have improved structuralintegrity and may better resist fracture during implantation by impact.Interbody spinal implants, as now taught, may therefore be used as adistractor during implantation.

Referring now to the drawing, in which like reference numbers refer tolike elements throughout the various figures that comprise the drawing,FIG. 1 shows a perspective view of a first embodiment of the interbodyspinal implant 1 especially well adapted for use in an ALIF procedure.The interbody spinal implant 1 includes a body having a top surface 10,a bottom surface 20, opposing lateral sides 30, and opposing anterior 40and posterior 50 portions. One or both of the top surface 10 and thebottom surface 20 has a roughened topography 80. Distinguish theroughened topography 80, however, from the disadvantageous teethprovided on the surfaces of some conventional devices.

Certain embodiments of the interbody spinal implant 1 are substantiallyhollow and have a generally oval-shaped transverse cross-sectional areawith smooth, rounded, or both smooth and rounded lateral sides andposterior-lateral corners. As used in this document, “substantiallyhollow” means at least about 33% of the interior volume of the interbodyspinal implant 1 is vacant. The implant 1 includes at least one verticalaperture 60 that extends the entire height of the implant body. Asillustrated in the top view of FIG. 2, the vertical aperture 60 furtherdefines a transverse rim 100 having a greater posterior portionthickness 55 than an anterior portion thickness 45.

In at least one embodiment, the opposing lateral sides 30 and theanterior portion 40 have a rim thickness of about 5 mm, while theposterior portion 50 has a rim thickness of about 7 mm. Thus, the rimposterior portion thickness 55 may allow for better stress sharingbetween the implant 1 and the adjacent vertebral endplates and helps tocompensate for the weaker posterior endplate bone. In certainembodiments, the transverse rim 100 has a generally large surface areaand contacts the vertebral endplate. The transverse rim 100 may act tobetter distribute contact stresses upon the implant 1, and henceminimize the risk of subsidence while maximizing contact with theapophyseal supportive bone. It is also possible for the transverse rim100 to have a substantially constant thickness (i.e., for the anteriorportion thickness 45 to be substantially the same as the posteriorportion thickness 55) or, in fact, for the posterior portion 50 to havea rim thickness less than that of the opposing lateral sides 30 and theanterior portion 40. Some studies have challenged the characterizationof the posterior endplate bone as weaker.

It is generally believed that the surface of an implant determines itsultimate ability to integrate into the surrounding living bone. Withoutbeing limited by theory, it is hypothesized that the cumulative effectsof at least implant composition, implant surface energy, and implantsurface roughness play a major role in the biological response to, andosteointegration of, an implant device. Thus, implant fixation maydepend, at least in part, on the attachment and proliferation ofosteoblasts and like-functioning cells upon the implant surface. Stillfurther, it appears that these cells attach more readily to relativelyrough surfaces rather than smooth surfaces. In this manner, a surfacemay be bioactive due to its ability to facilitate cellular attachmentand osteointegration. The surface roughened topography 80 may betterpromote the osteointegration of certain embodiments of the presentinvention. The surface roughened topography 80 may also better grip thevertebral endplate surfaces and inhibit implant migration upon placementand seating.

Accordingly, the implant 1 further includes the roughened topography 80on at least a portion of its top and bottom surfaces 10, 20 for grippingadjacent bone and inhibiting migration of the implant 1. The roughenedtopography 80 may be obtained through a variety of techniques including,without limitation, chemical etching, shot peening, plasma etching,laser etching, or abrasive blasting (such as sand or grit blasting). Inat least one embodiment, the interbody spinal implant 1 may be comprisedof titanium, or a titanium alloy, having the surface roughenedtopography 80. The surfaces of the implant 1 are preferably bioactive.

In a preferred embodiment of the present invention, the roughenedtopography 80 is obtained via the repetitive masking and chemical orelectrochemical milling processes described in U.S. Pat. No. 5,258,098;U.S. Pat. No. 5,507,815; U.S. Pat. No. 5,922,029; and U.S. Pat. No.6,193,762. Each of these patents is incorporated in this document byreference. Where the invention employs chemical etching, the surface isprepared through an etching process which utilizes the randomapplication of a maskant and subsequent etching of the metallicsubstrate in areas unprotected by the maskant. This etching process isrepeated a number of times as necessitated by the amount and nature ofthe irregularities required for any particular application. Control ofthe strength of the etchant material, the temperature at which theetching process takes place, and the time allotted for the etchingprocess allow fine control over the resulting surface produced by theprocess. The number of repetitions of the etching process can also beused to control the surface features.

By way of example, an etchant mixture of nitric acid (HNO₃) andhydrofluoric (HF) acid may be repeatedly applied to a titanium surfaceto produce an average etch depth of about 0.53 mm. Interbody spinalimplants, in accordance with preferred embodiments of the presentinvention, may be comprised of titanium, or a titanium alloy, having anaverage surface roughness of about 100 μm. Surface roughness may bemeasured using a laser profilometer or other standard instrumentation.

In another example, chemical modification of the titanium implantsurfaces can be achieved using HF and a combination of hydrochloric acidand sulfuric acid (HCl/H₂SO₄). In a dual acid etching process, the firstexposure is to HF and the second is to HCl/H₂SO₄. Chemical acid etchingalone of the titanium implant surface has the potential to greatlyenhance osteointegration without adding particulate matter (e.g.,hydroxyapatite) or embedding surface contaminants (e.g., gritparticles).

Certain embodiments of the implant 1 are generally shaped to reduce therisk of subsidence, and improve stability, by maximizing contact withthe apophyseal rim of the vertebral endplates. Embodiments may beprovided in a variety of anatomical footprints having a medial-lateralwidth ranging from about 32 mm to about 44 mm. Interbody spinalimplants, as now taught, generally do not require extensive supplementalor obstructive implant instrumentation to maintain the prepared discspace during implantation. Thus, the interbody spinal implant 1 andassociated implantation methods, according to presently preferredaspects of the present invention, allow for larger sized implants ascompared with the size-limited interbody spinal implants known in theart. This advantage allows for greater medial-lateral width andcorrespondingly greater contact with the apophyseal rim.

FIG. 3 depicts an anterior view, and FIG. 4 depicts a posterior view, ofan embodiment of the interbody spinal implant 1. As illustrated in FIGS.1 and 3, the implant 1 has an opening 90 in the anterior portion 40. Asillustrated in FIGS. 3 and 4, in one embodiment the posterior portion 50has a similarly shaped opening 90. In another embodiment, as illustratedin FIG. 1, only the anterior portion 40 has the opening 90 while theposterior portion 50 has an alternative opening 92 (which may have asize and shape different from the opening 90).

The opening 90 has a number of functions. One function is to facilitatemanipulation of the implant 1 by the caretaker. Thus, the caretaker mayinsert a surgical tool into the opening 90 and, through the engagementbetween the surgical tool and the opening 90, manipulate the implant 1.The opening 90 may be threaded to enhance the engagement.

FIG. 6 shows an exemplary surgical tool, specifically an implant holder2, to be used with certain embodiments of the interbody spinal implant1. Typically, the implant holder 2 has a handle 4 that the caretaker caneasily grasp and an end 6 that engages the opening 90. The end 6 may bethreaded to engage corresponding threads in the opening 90. The size andshape of the opening 90 can be varied to accommodate a variety of tools.Thus, although the opening 90 is substantially square as illustrated inFIGS. 1, 3, and 4, other sizes and shapes are feasible.

The implant 1 may further include at least one transverse aperture 70that extends the entire transverse length of the implant body. As shownin FIGS. 5A-5C, these transverse apertures 70 may provide improvedvisibility of the implant 1 during surgical procedures to ensure properimplant placement and seating, and may also improve post-operativeassessment of implant fusion. Still further, the substantially hollowarea defined by the implant 1 may be filled with cancellous autograftbone, allograft bone, DBM, porous synthetic bone graft substitute, BMP,or combinations of these materials (collectively, bone graft materials),to facilitate the formation of a solid fusion column within the spine ofa patient.

The anterior portion 40, or trailing edge, of the implant 1 ispreferably generally greater in height than the opposing posteriorportion 50. Accordingly, the implant 1 may have a lordotic angle tofacilitate sagittal alignment. The implant 1 may better compensate,therefore, for the generally less supportive bone found in the posteriorregions of the vertebral endplate. The posterior portion 50 of theinterbody implant 1, preferably including the posterior-lateral corners,may also be highly radiused, thus allowing for ease of implantation intothe disc space. Thus, the posterior portion 50 may have a generallyblunt nosed profile. The anterior portion 40 of the implant 1 may alsopreferably be configured to engage a delivery device, driver, or othersurgical tool (and, therefore, may have an opening 90).

As illustrated in FIG. 1, the anterior portion 40 of the implant 1 issubstantially flat. Thus, the anterior portion 40 provides a face thatcan receive impact from a tool, such as a surgical hammer, to force theimplant 1 into position. The implant 1 has a sharp edge 8 where theanterior portion 40 meets the top surface 10, where the anterior portion40 meets the bottom surface 20, or in both locations. The sharp edge oredges 8 function to resist pullout of the implant 1 once it is insertedinto position.

Certain embodiments of the present invention are particularly suited foruse during interbody spinal implant procedures (or vertebral bodyreplacement procedures) and may act as a final distractor duringimplantation, thus minimizing the instrument load upon the surgeon. Forexample, in such a surgical procedure, the spine may first be exposedvia an anterior approach and the center of the disc space identified.The disc space is then initially prepared for implant insertion byremoving vertebral cartilage. Soft tissue and residual cartilage maythen also be removed from the vertebral endplates.

Vertebral distraction may be performed using trials of various-sizedembodiments of the interbody spinal implant 1. The determinatively sizedinterbody implant 1 may then be inserted in the prepared disc space forfinal placement. The distraction procedure and final insertion may alsobe performed under fluoroscopic guidance. The substantially hollow areawithin the implant body may optionally be filled, at least partially,with bone fusion-enabling materials such as, without limitation,cancellous autograft bone, allograft bone, DBM, porous synthetic bonegraft substitute, BMP, or combinations of those materials. Such bonefusion-enabling material may be delivered to the interior of theinterbody spinal implant 1 using a delivery device mated with theopening 90 in the anterior portion 40 of the implant 1. Interbody spinalimplants 1, as now taught, are generally larger than those currentlyknown in the art, and therefore have a correspondingly larger hollowarea which may deliver larger volumes of fusion-enabling bone graftmaterial. The bone graft material may be delivered such that it fillsthe full volume, or less than the full volume, of the implant interiorand surrounding disc space appropriately.

As noted above, FIG. 1 shows a perspective view of one embodiment of thepresent invention, the interbody spinal implant 1, which is especiallywell adapted for use in an ALIF procedure. Other embodiments of thepresent invention are better suited for PLIF, TLIF, or cervical fusionprocedures. Specifically, FIGS. 9 and 10 show perspective views, fromthe front and rear, respectively, of an embodiment of an interbodyspinal implant 101 especially well adapted for use in a PLIF procedure.The interbody spinal implant 101 includes a body having a top surface110, a bottom surface 120, opposing lateral sides 130, and opposinganterior 140 and posterior 150 portions. One or both of the top surface110 and the bottom surface 120 has a roughened topography 180 forgripping adjacent bone and inhibiting migration of the implant 101.

Certain embodiments of the interbody spinal implant 101 aresubstantially hollow and have a generally rectangular shape with smooth,rounded, or both smooth and rounded lateral sides and anterior-lateralcorners. As best shown in FIG. 10, the anterior portion 140 may have atapered nose 142 to facilitate insertion of the implant 101. To furtherfacilitate insertion, the implant 101 has chamfers 106 at the corners ofits posterior portion 150. The chamfers 106 prevent the implant 101 fromcatching upon insertion, risking potential damage such as severednerves, while still permitting the implant 101 to have a sharp edge 108.

As illustrated in FIG. 9, the posterior portion 150 of the implant 101is substantially flat. Thus, the posterior portion 150 provides a facethat can receive impact from a tool, such as a surgical hammer, to forcethe implant 101 into position. The implant 101 has a sharp edge 108between the chamfers 106 where the posterior portion 150 meets the topsurface 110, where the posterior portion 150 meets the bottom surface120, or in both locations. The sharp edge or edges 108 function toresist pullout of the implant 101 once it is inserted into position.

The implant 101 includes at least one vertical aperture 160 that extendsthe entire height of the implant body. As illustrated in the top view ofFIG. 11, the vertical aperture 160 further defines a transverse rim 200.The size and shape of the vertical aperture 160 are carefully chosen toachieve a preferable design trade off for the particular applicationenvisioned for the implant 101. Specifically, the vertical aperture 160seeks to maximize the surface area of the top surface 110 and the bottomsurface 120 available proximate the anterior 140 and posterior 150portions while maximizing both radiographic visualization and access tothe bone graft material toward the center of the top 110 and bottom 120surfaces. Thus, the size and shape of the vertical aperture 160 arepredetermined by the application. By “predetermined” is meant determinedbeforehand, so that the predetermined size and shape are determined,i.e., chosen or at least known, before the implant 101 is selected forinsertion.

In the particular example shown in FIGS. 9-11, the width of the implant101 between the two lateral sides 130 is approximately 9 mm. The shapeof the vertical aperture 160 approximates, in cross section, that of anAmerican football. The center of the vertical aperture 160, whichdefines the maximum width of the vertical aperture 160, is about 5 mm.Thus, the rim thickness 200 on either side of the vertical aperture 160adjacent the center of the vertical aperture 160 is about 2 mm. Thesedimensions permit ample engagement between the bone graft materialcontained within the implant 101 and bone.

The vertical aperture 160 tapers from its center to its ends along alongitudinal distance of about 7.75 mm (thus, the total length of thevertical aperture 160 is about 15.5 mm). This shape leaves intact muchof the rim thickness 200 in the areas around the ends of the verticalaperture 160. These areas may allow for better stress sharing betweenthe implant 101 and the adjacent vertebral endplates. Thus, thetransverse rim 200 has a generally large surface area and contacts thevertebral endplate.

As illustrated in FIG. 9, the implant 101 has an opening 190 in theposterior portion 150. The opening 190 has a number of functions. Onefunction is to facilitate manipulation of the implant 101 by thecaretaker. Thus, the caretaker may insert a surgical tool (FIG. 6 showsan exemplary surgical tool, the implant holder 2) into the opening 190and, through the engagement between the surgical tool and the opening190, manipulate the implant 101. The opening 190 may be threaded toenhance the engagement.

The implant 101 may also have an Implant Holding Feature (IHF) 194instead of or in addition to the opening 190. As illustrated in FIG. 9,the IHF 194 is located proximate the opening 190 in the posteriorportion 150. In this particular example, the IHF 194 is a U-shapednotch. Like the opening 190, the IHF 194 has a number of functions, oneof which is to facilitate manipulation of the implant 101 by thecaretaker. Other functions of the opening 190 and the IHF 194 are toincrease visibility of the implant 101 during surgical procedures and toenhance engagement between bone graft material and adjacent bone.

The implant 101 may further include at least one transverse aperture170. Like the vertical aperture 160, the size and shape of thetransverse aperture 170 are carefully chosen (and predetermined) toachieve a preferable design trade off for the particular applicationenvisioned for the implant 101. Specifically, the transverse aperture170 should have minimal dimensions to maximize the strength andstructural integrity of the implant 101. On the other hand, thetransverse aperture 70 should have maximum dimensions to (a) improve thevisibility of the implant 101 during surgical procedures to ensureproper implant placement and seating, and to improve post-operativeassessment of implant fusion, and (b) to facilitate engagement betweenbone graft material and adjacent bone. The substantially hollow areadefined by the implant 101 may be filled with bone graft materials tofacilitate the formation of a solid fusion column within the spine of apatient.

As shown in FIGS. 9 and 10, the transverse aperture 170 extends theentire transverse length of the implant body and nearly the entireheight of the implant body. Thus, the size and shape of the transverseaperture 170 approach the maximum possible dimensions for the transverseaperture 170. Like FIG. 10, FIG. 12 shows a perspective view from therear of the interbody spinal implant 101. FIG. 12 highlights, however,an alternative transverse aperture 170.

As illustrated in FIG. 12, the transverse aperture 170 is broken intotwo, separate sections by an intermediate wall 172. Thus, the dimensionsof the transverse aperture 170 shown in FIG. 12 are much smaller thanthose for the transverse aperture 170 shown in FIG. 10. The section ofthe transverse aperture 170 proximate the IHF 194 is substantiallyrectangular in shape; the other section of the transverse aperture 170has the shape of a curved arch. Other shapes and dimensions are suitablefor the transverse aperture 170. In particular, all edges of thetransverse aperture 170 may be rounded, smooth, or both. Theintermediate wall 172 may be made of the same material as the remainderof the implant 101 (e.g., metal), or it may be made of another material(e.g., PEEK) to form a composite implant 101. The intermediate wall 172may offer one or more of several advantages, including reinforcement ofthe implant 101 and improved bone graft containment.

The embodiment of the present invention illustrated in FIGS. 9-12 isespecially well suited for a PLIF surgical procedure. TLIF surgery isdone through the posterior (rear) part of the spine and is essentiallylike an extended PLIF procedure. The TLIF procedure was developed inresponse to some of the technical problems encountered with a PLIFprocedure. The main difference between the two spine fusion proceduresis that the TLIF approach to the disc space is expanded by removing oneentire facet joint; a PLIF procedure is usually done on both sides byonly taking a portion of each of the paired facet joints.

By removing the entire facet joint, visualization into the disc space isimproved and more disc material can be removed. Such removal should alsoprovide for less nerve retraction. Because one entire facet is removed,the TLIF procedure is only done on one side: removing the facet jointson both sides of the spine would result in too much instability. Withincreased visualization and room for dissection, one or both of a largerimplant and more bone graft can be used in the TLIF procedure.Theoretically, these advantages can allow the spine surgeon to distractthe disc space more and realign the spine better (re-establish thenormal lumbar lordosis).

Although the TLIF procedure offers some improvements over a PLIFprocedure, the anterior approach in most cases still provides the bestvisualization, most surface area for healing, and the best reduction ofany of the approaches to the disc space. These advantages must beweighed, however, against the increased morbidity (e.g., unwantedaftereffects and postoperative discomfort) of a second incision.Probably the biggest determinate in how the disc space is approached isthe comfort level that the spine surgeon has with an anterior approachfor the spine fusion surgery. Not all spine surgeons are comfortablewith operating around the great vessels (aorta and vena cava) or haveaccess to a skilled vascular surgeon to help them with the approach.Therefore, choosing one of the posterior approaches for the spine fusionsurgery is often a more practical solution.

The embodiment of the present invention illustrated in FIGS. 13-15 isespecially well suited when the spine surgeon elects a TLIF procedure.Many of the features of the implant 101 a illustrated in FIGS. 13-15 arethe same as those of the implant 101 illustrated in FIGS. 9-12.Therefore, these features are given the same reference numbers, with theaddition of the letter “a,” and are not described further.

There are several differences, however, between the two embodiments. Forexample, unlike the substantially rectangular shape of the implant 101,the implant 101 a has a curved shape. Further, the chamfers 106 andsharp edges 108 of the implant 101 are replaced by curves or roundededges for the implant 101 a. Still further, the TLIF procedure oftenpermits use of a larger implant 101 a which, in turn, may affect thesize and shape of the predetermined vertical aperture 160 a.

The effect of the larger (relative to the implant 101) implant 101 a isshown in FIG. 14, which illustrates a top view of the implant 101 a. Thesubstantially constant 9 mm width of the transverse rim 200 of theimplant 101 is replaced with a larger, curved transverse rim 200 a. Thewidth of the transverse rim 200 a is 9 mm in the regions adjacent theanterior 140 a and posterior 150 a portions. That width graduallyincreases to 11 mm, however, near the center of the transverse rim 200a. The additional real estate provided by the transverse rim 200 a(relative to the transverse rim 200) allows the shape of the verticalaperture 160 a to change, in cross section, from approximating afootball to approximating a boomerang. Maintaining the thickness of thetransverse rim 200 a on either side of the vertical aperture 160 aadjacent the center of the vertical aperture 160 a at about 2 mm,similar to the dimensions of the implant 101, the center of the verticalaperture 160 a, which defines the maximum width of the vertical aperture160 a, is increased (from 5 mm for the implant 101) to about 7 mm.

The implant 101 a may also have a lordotic angle to facilitatealignment. As illustrated in FIG. 14, the lateral side 130 a depicted atthe top of the implant 101 a is preferably generally greater in heightthan the opposing lateral side 130 a. Therefore, the implant 101 a maybetter compensate for the generally less supportive bone found incertain regions of the vertebral endplate.

As shown in FIG. 13, the transverse aperture 170 a extends the entiretransverse length of the implant body and nearly the entire height ofthe implant body. FIG. 15 highlights an alternative transverse aperture170 a. As illustrated in FIG. 15, the transverse aperture 170 a isbroken into two, separate sections by an intermediate wall 172 a. Thus,the dimensions of the transverse aperture 170 a shown in FIG. 15 aremuch smaller than those for the transverse aperture 170 a shown in FIG.13. The two sections of the alternative transverse aperture 170 a areeach illustrated as substantially rectangular in shape and extendingnearly the entire height of the implant body; other sizes and shapes arepossible for one or both sections of the alternative transverse aperture170 a.

The intermediate wall 172 a may be made of the same material as theremainder of the implant 101 a (e.g., metal), or it may be made ofanother material (e.g., PEEK) to form a composite implant 101 a. It isalso possible to extend the intermediate wall 172 a, whether made ofmetal, PEEK, ultra-high molecular weight polyethylene (UHMWPE), oranother material, to eliminate entirely the transverse aperture 170 a.Given the reinforcement function of the intermediate wall 172 a, thelength of the vertical aperture 160 a can be extended (as shown in FIG.15) beyond the top surface 110 a and into the anterior portion 140 a ofthe implant 101 a.

Also important is that the top surface 110 a of the implant 101 a shownin FIG. 14 differs from the top surface 110 a of the implant 101 a shownin FIGS. 13 and 15 in that the former does not include the roughenedtopography 180 a of the latter. This difference permits the implant 101a, at least for certain applications, to be made entirely of a non-metalmaterial. Suitable materials of construction for the implant 101 a ofsuch a design (which would not be a composite) include PEEK, hedrocel,UHMWPE, other radiolucent soft plastics, and additional materials aswould be known to an artisan.

Another embodiment of a composite implant 101 a is illustrated in FIGS.16 and 17. FIG. 16 shows a perspective view from the side of onecomponent of the composite implant 101 a: an all plastic body 152. Thebody 152 may preferably be injection molded. PEEK is a suitable materialfor the body 152. In order to retain the advantages of a metal surface,including strength and an acid-etched roughened topography 180 a, asecond component of the composite implant 101 a is provided: one or moremetal strips or plates 162. The plates 162 may be provided on the topsurface 110 a, on the bottom surface 120 a, or on both surfaces 110 aand 120 a.

Thus, the composite implant 101 a combines the benefits of two, separatecomponents: a body 152 and a plate 162. The composite structure ofimplant 101 a advantageously permits the engineering designer of theimplant 101 a to balance the mechanical characteristics of the overallimplant 101 a. This allows the implant 101 a to achieve the bestbalance, for example, of strength, resistance to subsidence, and stresstransfer to bone graft. Moreover, although it is a relatively widedevice designed to engage the ends of the vertebrae, the implant 101 acan be inserted with minimal surgical modification. This combination ofsize and minimal surgical modification is advantageous.

The two components that form the composite implant 101 a must beconnected. As illustrated in FIG. 16, the body 152 of the compositeimplant 101 a has a recessed upper surface 154. The recessed uppersurface 154 is recessed below the top surface 110 a of the compositeimplant 101 a by an amount corresponding to the thickness of the plate162 that will be installed over the recessed upper surface 154 to createa substantially flat top surface 110 a. FIG. 17 is a top view of thecomposite interbody spinal implant 101 a with the plate 162 installed.As illustrated in FIG. 17, the plate 162 has a roughened topography 180a. A corresponding second plate 162 may be installed on the bottomsurface 120 a of the composite implant 101 a.

Any suitable connection mechanism, as would be known to an artisan, willsuffice to install the plate 162 on the recessed upper surface 154 ofthe body 152. One connection mechanism is illustrated in FIGS. 16 and18. FIG. 16 shows that a plurality of holes 156 are provided in therecessed upper surface 154 of the body 152. The holes 156 receive acorresponding plurality of legs 164 on the plate 162. The legs 164 arepositioned on the plate 162 so that, when the plate 162 is installedover the recessed upper surface 154 of the body 152, each of the legs164 engages one of the holes 156. Preferably, the legs 164 are integralwith the remainder of the plate 162. By “integral” is meant a singlepiece or a single unitary part that is complete by itself withoutadditional pieces, i.e., the part is of one monolithic piece formed as aunit with another part.

As shown in FIG. 18, the legs 164 may be configured to prevent them fromexiting the holes 156. Thus, as shown, the legs 164 have a toothedperiphery. If the plate 162 is metal and the body 152 is plastic, thebody 152 may be injection molded around the legs 164 of the plate 162.In some applications, for example were the body 152 and the plate 162both made of metal, it may be possible to provide corresponding threadson the legs 164 and holes 156.

The embodiments of the present invention described above are best suitedfor one or more of the ALIF, PLIF, and TLIF surgical procedures. Anotherembodiment of the present invention is better suited for cervical fusionprocedures. This embodiment is illustrated in FIGS. 19 and 20 as theinterbody spinal implant 201.

Because there is not a lot of disc material between the vertebral bodiesin the cervical spine, the discs are usually not very large. The spaceavailable for the nerves is also not that great, however, which meansthat even a small cervical disc herniation may impinge on the nerve andcause significant pain. There is also less mechanical load on the discsin the cervical spine as opposed to the load that exists lower in thespine. Among others, these differences have ramifications for the designof the implant 201.

The implant 201 is generally smaller in size than the other implantembodiments. In addition, the lower mechanical load requirements imposedby the cervical application typically render a composite implantunnecessary. Therefore, the implant 201 is generally made entirely ofmetal (e.g., titanium) and devoid of other materials (e.g., PEEK).

With specific reference to FIG. 19, the implant 201 includes a bodyhaving a top surface 210, a bottom surface 220, opposing lateral sides230, and opposing anterior 240 and posterior 250 portions. One or bothof the top surface 210 and the bottom surface 220 has a roughenedtopography 280 for gripping adjacent bone and inhibiting migration ofthe implant 201. The implant 201 is substantially hollow and has agenerally oval shape with smooth, rounded, or both smooth and roundededges.

The implant 201 includes at least one vertical aperture 260 that extendsthe entire height of the implant body. The vertical aperture 260 furtherdefines a transverse rim 300. The size and shape of the verticalaperture 260 are carefully chosen to achieve a preferable design tradeoff for the particular application envisioned for the implant 201.Specifically, the vertical aperture 260 seeks to maximize the surfacearea of the top surface 210 and the bottom surface 220, to allow forbetter stress sharing between the implant 201 and the adjacent vertebralendplates, while maximizing access to the bone graft material providedwithin the implant 201. Thus, the size and shape of the verticalaperture 260 are predetermined by the application.

As illustrated in FIG. 19, the implant 201 has an opening 290 in theposterior portion 250. The opening 290 has a number of functions. Onefunction is to facilitate manipulation of the implant 201 by thecaretaker. Thus, the caretaker may insert a surgical tool (FIG. 6 showsan exemplary surgical tool, the implant holder 2) into the opening 290and, through the engagement between the surgical tool and the opening290, manipulate the implant 201. The opening 290 may be threaded toenhance the engagement.

The implant 201 may further include at least one transverse aperture270. Like the vertical aperture 260, the size and shape of thetransverse aperture 270 are carefully chosen (and predetermined) toachieve a preferable design trade off for the particular applicationenvisioned for the implant 201. For example, as shown in FIG. 19, thetransverse aperture 270 may extend the entire transverse length of theimplant body and nearly the entire height of the implant body. Thus, thesize and shape of the transverse aperture 270 approach the maximumpossible dimensions for the transverse aperture 270.

As illustrated in FIG. 19, the implant 201 may be provided with a solidrear wall 242. The rear wall 242 extends the entire width of the implantbody and nearly the entire height of the implant body. Thus, the rearwall 242 essentially closes the anterior portion 240 of the implant 201.The rear wall 242 may offer one or more of several advantages, includingreinforcement of the implant 201 and improved bone graft containment. Inthe cervical application, it may be important to prevent bone graftmaterial from entering the spinal canal.

Alternative shapes for the implant 201 are possible. As illustrated inFIG. 20, for example, the implant 201 may have a generally box shapewhich gives the implant 201 increased cortical bone coverage. Like theimplant 201 shown in FIG. 19, the implant 201 shown in FIG. 20 has acurved transverse rim 300 in the area of the anterior portion 240. Theshape of the posterior portion 250 of the implant 201 is substantiallyflat, however, and the shape of the transverse rim 300 in the area ofthe posterior portion 250 is substantially square. Thus, the posteriorportion 250 provides a face that can receive impact from a tool, such asa surgical hammer, to force the implant 201 into position.

The implant 201 may also have a lordotic angle to facilitate alignment.As illustrated in FIGS. 19 and 20, the anterior portion 240 ispreferably generally greater in height than the posterior portion 250.Therefore, the implant 201 may better compensate for the generally lesssupportive bone found in certain regions of the vertebral endplate. Asan example, four degrees of lordosis may be built into the implant 201to help restore balance to the spine.

Certain embodiments of the implant 1, 101, 101 a, and 201 are generallyshaped (i.e., made wide) to maximize contact with the apophyseal rim ofthe vertebral endplates. They are designed to be impacted between theendplates, with fixation to the endplates created by an interference fitand annular tension. Thus, the implants 1, 101, 101 a, and 201 areshaped and sized to spare the vertebral endplates and leave intact thehoop stress of the endplates. A wide range of sizes are possible tocapture the apophyseal rim, along with a broad width of the peripheralrim, especially in the posterior region. It is expected that suchdesigns will lead to reduced subsidence. As much as seven degrees oflordosis (or more) may be built into the implants 1, 101, 101 a, and 201to help restore cervical balance.

When endplate-sparing spinal implant 1, 101, 101 a, and 201 seats in thedisc space against the apophyseal rim, it should still allow fordeflection of the endplates like a diaphragm. This means that,regardless of the stiffness of the spinal implant 1, 101, 101 a, and201, the bone graft material inside the spinal implant 1, 101, 101 a,and 201 receives load, leading to healthy fusion. The vertical load inthe human spine is transferred though the peripheral cortex of thevertebral bodies. By implanting an apophyseal-supporting inter-bodyimplant 1, 101, 101 a, and 201, the natural biomechanics may be betterpreserved than for conventional devices. If this is true, the adjacentvertebral bodies should be better preserved by the implant 1, 101, 101a, and 201, hence reducing the risk of adjacent segment issues.

In addition, the dual-acid etched roughened topography 80, 180, 180 a,and 280 of the top surface 30, 130, 130 a, and 230 and the bottomsurface 40, 140, 140 a, and 240 along with the broad surface area ofcontact with the end-plates, is expected to yield a high pull-out forcein comparison to conventional designs. As enhanced by the sharp edges 8and 108, a pull-out strength of up to 3,000 nt may be expected. Theroughened topography 80, 180, 180 a, and 280 creates a biological bondwith the end-plates over time, which should enhance the quality offusion to the bone. Also, the in-growth starts to happen much earlierthan the bony fusion. The center of the implant 1, 101, 101 a, and 201remains open to receive bone graft material and enhance fusion.Therefore, it is possible that patients might be able to achieve a fullactivity level sooner than for conventional designs.

The spinal implant 1, 101, 101 a, and 201 according to the presentinvention offers several advantages relative to conventional devices.Such conventional devices include, among others, ring-shaped cages madeof allograft bone material, threaded titanium cages, and ring-shapedcages made of PEEK or carbon fiber. Several of the advantages aresummarized with respect to each conventional device, in turn, asfollows.

1. Advantages Over Allograft Bone Material Cages

The spinal implant 1, 101, 101 a, and 201 is easier to use thanring-shaped cages made of allograft bone material. For example, it iseasier to prepare the graft bed, relative to the allograft cage, for thespinal implant 1, 101, 101 a, and 201. And ring allograft cagestypically are not sufficiently wide to be implanted on the apophasis.The spinal implant 1, 101, 101 a, and 201 offers a large internal areafor bone graft material and does not require graft preparation, cutting,or trimming. The central aperture 60, 160, 160 a, and 260 of the spinalimplant 1, 101, 101 a, and 201 can be filled with cancellous allograft,porous synthetic bone graft substitute (such as the material offered byOrthovita, Inc., Malvern, Pa., under the Vitoss trademark), or BMP. Theprocess of healing the bone can proceed by intra-membranous ossificationrather than the much slower process of enchondral ossification.

The spinal implant 1, 101, 101 a, and 201 is generally stronger thanallograft cages. In addition, the risk of osteolysis (or, moregenerally, disease transmission) is minimal with the spinal implant 1,101, 101 a, and 201 because titanium is osteocompatible. The titanium ofthe spinal implant 1, 101, 101 a, and 201 is unaffected by BMP; therehave been reports that BMP causes resorption of allograft bone.

2. Advantages Over Threaded Titanium Cages

In contrast to conventional treaded titanium cages, which offer littlebone-to-bone contact (about 9%), the spinal implant 1, 101, 101 a, and201 has a much higher bone-to-bone contact area and commensuratelylittle metal-to-bone interface. Unlike threaded titanium cages whichhave too large a diameter, the spinal implant 1, 101, 101 a, and 201 canbe relatively easily used in “tall” disc spaces. The spinal implant 1,101, 101 a, and 201 can also be used in either a “stand alone” manner incollapsed discs or as an adjunct to a 360-degree fusion providingcervical column support.

The spinal implant 1, 101, 101 a, and 201 offers safety advantages overconventional threaded titanium cages. The spinal implant 1, 101, 101 a,and 201 is also easier to implant, avoiding the tubes necessary toinsert some conventional cages, and easier to center. Without having toput a tube into the disc space, the vein can be visualized by both thespine surgeon and the vascular surgeon while working with the spinalimplant 1, 101, 101 a, and 201. Anterior-posterior (AP) fluoroscopy caneasily be achieved with trial before implanting the spinal implant 1,101, 101 a, and 201, ensuring proper placement. The smooth and roundededges of the spinal implant 1, 101, 101 a, and 201 facilitate insertionand enhance safety. No reaming of the endplate, which weakens theinterface between the endplate and the cage, is necessary for the spinalimplant 1, 101, 101 a, and 201. Therefore, no reamers or taps aregenerally needed to insert and position the spinal implant 1, 101, 101a, and 201.

3. Advantages Over PEEK/Carbon Fiber Cages

Cages made of PEEK or carbon fiber cannot withstand the high impactforces needed for implantation, especially in a collapsed disc orspondylolisthesis situation, without secondary instruments. In contrast,the spinal implant 1, 101, 101 a, and 201 avoids the need for secondaryinstruments. Moreover, relative to PEEK or carbon fiber cages, thespinal implant 1, 101, 101 a, and 201 provides better distractionthrough endplate sparing and being designed to be implanted on theapophysis (the bony protuberance of the human spine). The titanium ofthe top surface 10, 110, 110 a, and 210 and the bottom plate 20, 120,120 a, and 220 of the spinal implant 1, 101, 101 a, and 201 binds tobone with a mechanical (knawling) and a chemical (a hydrophilic) bond.In contrast, bone repels PEEK and such incompatibility can lead tolocked pesudoarthrosis.

Example Surgical Methods

The following examples of surgical methods are included to more clearlydemonstrate the overall nature of the invention. These examples areexemplary, not restrictive, of the invention.

Certain embodiments of the present invention are particularly suited foruse during interbody spinal implant procedures currently known in theart. For example, the disc space may be accessed using a standard miniopen retroperitoneal laparotomy approach. The center of the disc spaceis located by AP fluoroscopy taking care to make sure the pedicles areequidistant from the spinous process. The disc space is then incised bymaking a window in the annulus for insertion of certain embodiments ofthe spinal implant 1, 101, 101 a, and 201 (a 32 or 36 mm window in theannulus is typically suitable for insertion). The process according tothe present invention minimizes, if it does not eliminate, the cuttingof bone. The endplates are cleaned of all cartilage with a curette,however, and a size-specific rasp (or broach) may then be used.

FIG. 8 shows an exemplary rasp 14 used during certain methods ofimplantation. Typically, either a 32 mm or a 36 mm rasp 14 is used. Asingle rasp 14 is used to remove a minimal amount of bone. A lateralc-arm fluoroscopy can be used to follow insertion of the rasp 14 in theposterior disc space. The smallest height rasp 14 that touches bothendplates (e.g., the superior and inferior endplates) is first chosen.After the disc space is cleared of all soft tissue and cartilage,distraction is then accomplished by using distractors (also calledimplant trials or distraction plugs). It is usually possible to distract2-3 mm higher than the rasp 14 that is used because the disk space iselastic.

FIG. 7 shows an exemplary distractor 12 used during certain methods ofimplantation. The implant trials, or distractors 12, are solid polishedblocks which have a peripheral geometry identical to that of the implant1, 101, 101 a, and 201. These distractor blocks may be made in variousheights to match the height of the implant 1, 101, 101 a, and 201. Thedisc space is adequately distracted by sequentially expanding it withdistractors 12 of progressively increasing heights. The distractor 12 isthen left in the disc space and the centering location may be checked byplacing the back into the AP position. If the location is confirmed ascorrect (e.g., centered), the c-arm is turned back into the lateralposition. The spinal implant 1, 101, 101 a, and 201 is filled withautologous bone graft or bone graft substitute. The distractor 12 isremoved and the spinal implant 1, 101, 101 a, and 201 is inserted underc-arm fluoroscopy visualization. The process according to the presentinvention does not use a secondary distractor; rather, distraction ofthe disc space is provided by the spinal implant 1, 101, 101 a, and 201itself (i.e., the implant 1, 101, 101 a, and 201 itself is used as adistractor).

Use of a size-specific rasp 14, as shown in FIG. 8, preferably minimizesremoval of bone, thus minimizing impact to the natural anatomical arch,or concavity, of the vertebral endplate while preserving much of theapophyseal rim. Preservation of the anatomical concavity is particularlyadvantageous in maintaining biomechanical integrity of the spine. Forexample, in a healthy spine, the transfer of compressive loads from thevertebrae to the spinal disc is achieved via hoop stresses acting uponthe natural arch of the endplate. The distribution of forces, andresultant hoop stress, along the natural arch allows the relatively thinshell of subchondral bone to transfer large amounts of load.

During traditional fusion procedures, the vertebral endplate naturalarch may be significantly removed due to excessive surface preparationfor implant placement and seating. This is especially common where theimplant is to be seated near the center of the vertebral endplate or theimplant is of relatively small medial-lateral width. Breaching thevertebral endplate natural arch disrupts the biomechanical integrity ofthe vertebral endplate such that shear stress, rather than hoop stress,acts upon the endplate surface. This redistribution of stresses mayresult in subsidence of the implant into the vertebral body.

Preferred embodiments of the present surgical method minimize endplatebone removal on the whole, while still allowing for some removal alongthe vertebral endplate far lateral edges where the subchondral bone isthickest. Still further, certain embodiments of the present interbodyspinal implant 1, 101, 101 a, and 201 include smooth, rounded, andhighly radiused posterior portions and lateral sides which may minimizeextraneous bone removal for endplate preparation and reduce localizedstress concentrations. Thus, interbody surgical implants 1, 101, 101 a,and 201 and methods of using them, as now taught, are particularlyuseful in preserving the natural arch of the vertebral endplate andminimizing the chance of implant subsidence.

Because the endplates are spared during the process of inserting thespinal implant 1, 101, 101 a, and 201, hoop stress of the inferior andsuperior endplates is maintained. Spared endplates allow the transfer ofaxial stress to the apophasis. Endplate flexion allows the bone graftplaced in the interior of the spinal implant 1, 101, 101 a, and 201 toaccept and share stress transmitted from the endplates. In addition,spared endplates minimize the concern that BMP might erode thecancellous bone.

Interbody spinal implants 1, 101, 101 a, and 201 of the presentinvention are durable and can be impacted between the endplates withstandard instrumentation. Therefore, certain embodiments of the presentinvention may be used as the final distractor during implantation. Inthis manner, the disc space may be under-distracted (e.g., distracted tosome height less than the height of the interbody spinal implant 1, 101,101 a, and 201) to facilitate press-fit implantation. Further, certainembodiments of the current invention having a smooth and roundedposterior portion (and lateral sides) may facilitate easier insertioninto the disc space. Still further, those embodiments having a surfaceroughened topography 80, 180, 180 a, and 280, as now taught, may lessenthe risk of excessive bone removal during distraction as compared toimplants having teeth, ridges, or threads currently known in the arteven in view of a press-fit surgical distraction method. Nonetheless,once implanted, the interbody surgical implants 1, 101, 101 a, and 201,as now taught, may provide secure seating and prove difficult to remove.Thus, certain embodiments of the present interbody spinal implant 1,101, 101 a, and 201 may maintain a position between the vertebralendplates due, at least in part, to resultant annular tensionattributable to press-fit surgical implantation and, post-operatively,improved osteointegration at the top surface 10, 110, 110 a, and 210,the bottom surface 20, 120, 120 a, and 220, or both top and bottomsurfaces.

As previously mentioned, surgical implants and methods, as now taught,tension the vertebral annulus via distraction. These embodiments andmethods may also restore spinal lordosis, thus improving sagittal andcoronal alignment. Implant systems currently known in the art requireadditional instrumentation, such as distraction plugs, to tension theannulus. These distraction plugs require further tertiaryinstrumentation, however, to maintain the lordotic correction duringactual spinal implant insertion. If tertiary instrumentation is notused, then some amount of lordotic correction may be lost upondistraction plug removal. Interbody spinal implants 1, 101, 101 a, and201, according to certain embodiments of the present invention, areparticularly advantageous in improving spinal lordosis without the needfor tertiary instrumentation, thus reducing the instrument load upon thesurgeon. This reduced instrument load may further decrease thecomplexity, and required steps, of the implantation procedure.

Certain embodiments of the spinal implants 1, 101, 101 a, and 201 mayalso reduce deformities (such as isthmic spondylolythesis) caused bydistraction implant methods. Traditional implant systems requiresecondary or additional instrumentation to maintain the relativeposition of the vertebrae or distract collapsed disc spaces. Incontrast, interbody spinal implants 1, 101, 101 a, and 201, as nowtaught, may be used as the final distractor and thus maintain therelative position of the vertebrae without the need for secondaryinstrumentation.

Certain embodiments collectively comprise a family of implants, eachhaving a common design philosophy. These implants and the associatedsurgical technique have been designed to address the ten, separatechallenges associated with the current generation of traditionalanterior spinal fusion devices listed above in the Background section ofthis document. Each of these challenges is addressed in turn and in theorder listed above.

1. End-Plate Preparation

Embodiments of the present invention allow end-plate preparation withcustom-designed rasps 14. These rasps 14 have a geometry matched withthe geometry of the implant. The rasps 14 conveniently remove cartilagefrom the endplates and remove minimal bone, only in the postero-lateralregions of the vertebral end-plates. It has been reported in theliterature that the end-plate is the strongest in postero-lateralregions.

2. Implant Difficulty

After desired annulotomy and discectomy, embodiments of the presentinvention first adequately distract the disc space by inserting (throughimpaction) and removing sequentially larger sizes of very smoothdistractors, which have size matched with the size of the availableimplants 1, 101, 101 a, and 201. Once adequate distraction is achieved,the surgeon prepares the end-plate with a size-specific rasp 14. Thereis no secondary instrumentation required to keep the disc spacedistracted while the implant 1, 101, 101 a, and 201 is inserted, as theimplant 1, 101, 101 a, and 201 has sufficient mechanical strength thatit is impacted into the disc space. In fact, the height of the implant1, 101, 101 a, and 201 is about 1 mm greater than the height of the rasp14 used for end-plate preparation, to create some additional tension inthe annulus by implantation, which creates a stable implant construct inthe disc space.

The implant geometry has features which allow it to be implanted via anyone of an anterior, antero-lateral, or lateral approach, providingtremendous intra-operative flexibility of options. The implant 1, 101,101 a, and 201 is designed such that all the impact loads are appliedonly to the titanium part of the construct. Thus, the implant 1, 101,101 a, and 201 has adequate strength to allow impact. The sides of theimplant 1, 101, 101 a, and 201 have smooth surfaces to allow for easyimplantation and, specifically, to prevent “binding” of the implant 1,101, 101 a, and 201 to soft tissues during implantation.

3. Materials of Construction

The present invention encompasses a number of different implants 1, 101,101 a, and 201, including a one-piece, titanium-only implant and acomposite implant formed of top and bottom plates 162 (components) madeout of titanium. The surfaces exposed to the vertebral body are dualacid etched to allow for bony in-growth over time, and to provideresistance against expulsion. The top and bottom titanium plates 162 areassembled together with the implant body 152 that is injection moldedwith PEEK. The net result is a composite implant 101 a that hasengineered stiffness for its clinical application. The axial load isborne by the PEEK component of the construct.

It is believed that an intact vertebral end-plate deflects like adiaphragm under axial compressive loads generated due to physiologicactivities. If a spinal fusion implant is inserted in the prepared discspace via a procedure which does not destroy the end-plates, and if theimplant contacts the end-plates only peripherally, the central dome ofthe end-plates can still deflect under physiologic loads. Thisdeflection of the dome can pressurize the bone graft material packedinside the spinal implant, hence allowing it to heal naturally. Theimplant 1, 101, 101 a, and 201 designed according to certain embodimentsof the present invention allows the vertebral end-plate to deflect andallows healing of the bone graft into fusion.

4. Implant Expulsion

Certain faces of the implant 1, 101, 101 a, and 201 according to certainembodiments of the present invention have sharp edges 8, 180. Theseedges 8, 180 tend to dig “into” the end-plates slightly and help toresist expulsion. The top and bottom surfaces of the implant are madeout of titanium and are dual acid etched. The dual acid etching processcreates a highly roughened texture on these surfaces, which generatestremendous resistance to expulsion. The width of these dual acid etchedsurfaces is very broad and creates a large area of contact with thevertebral end-plates, further increasing the resistance to expulsion.

5. Implant Subsidence

The implant 1, 101, 101 a, and 201 according to certain embodiments ofthe present invention has a large foot-print, and offers several sizes.Because there is no secondary instrument required to maintaindistraction during implantation, all the medial-lateral (ML) exposure isavailable as implantable ML width of the implant. This feature allowsthe implant to contact the vertebral end-plates at the peripheralapophyseal rim, where the end-plates are the strongest and least likelyto subside.

Further, there are no teeth on the top and bottom surfaces (teeth cancreate stress risers in the end-plate, encouraging subsidence). Exceptfor certain faces, all the implant surfaces have heavily rounded edges,creating a low stress contact with the end-plates. The wide rim of thetop and bottom surfaces, in contact with the end-plates, creates alow-stress contact due to the large surface area. Finally, the implantconstruct has an engineered stiffness to minimize the stiffness mismatchwith the vertebral body which it contacts.

6. Insufficient Room for Bone Graft

As mentioned, the implant 1, 101, 101 a, and 201 according to certainembodiments of the present invention has a large foot-print. Inaddition, titanium provides high strength for a small volume. Incombination, the large foot-print along with the engineered use oftitanium allows for a large volume of bone graft to be placed inside theimplant.

7. Stress Shielding

As stated above, it is believed that an intact vertebral end-platedeflects like a diaphragm under axial compressive loads generated due tophysiologic activities. If a spinal fusion implant is inserted in theprepared disc space via a procedure which does not destroy theend-plate, and if the implant contacts the end-plates only peripherally,the central dome of the end-plates can still deflect under physiologicloads. This deflection of the dome can pressurize the bone graftmaterial packed inside the spinal implant, hence allowing it to healnaturally. The implant 1, 101, 101 a, and 201 according to certainembodiments of the present invention allows the vertebral end-plate todeflect and facilitates healing of the bone graft into fusion.

8. Lack of Implant Incorporation with Vertebral Bone

The top and bottom surfaces of the implant 1, 101, 101 a, and 201according to certain embodiments of the present invention are made oftitanium and are dual acid etched. The dual acid etched surfacetreatment of titanium allows in-growth of bone to the surfaces. Hence,the implant 1, 101, 101 a, and 201 is designed to incorporate with thevertebral bone over time. It may be that the in-growth happens soonerthan fusion. If so, there may be an opportunity for the patients treatedwith the implant 1, 101, 101 a, and 201 of the present invention toreturn to normal activity levels sooner than currently recommended bystandards of care.

9. Limitations on Radiographic Visualization

Even the titanium-only embodiment of the present invention has beendesigned with large windows to allow for radiographic evaluation offusion, both through AP and lateral X-rays. The composite implant 101 aminimizes the volume of titanium, and localizes it to the top and bottomsurfaces. The rest of the implant 101 a is made of PEEK which isradiolucent and allows for free radiographic visualization.

10. Cost of Manufacture and Inventory

The cost to manufacture a single implant 1, 101, 101 a, and 201according to the present invention is comparable to the cost tomanufacture commercially available products. But a typical implant setfor a conventional device can have three foot-prints and ten heights foreach foot-print. Therefore, to produce one set, the manufacturer has tomake thirty different setups if the implants are machined. In contrast,for the composite embodiment according to certain embodiments of thepresent invention, the manufacturer will have to machine only three setsof metal plates, which is six setups. The PEEK can be injection moldedbetween the metal plates separated by the distance dictated by theheight of the implant 101 a. Once the injection molds are made, thesubsequent cost of injection molding is considerably less as compared tomachining. This feature of the present invention can lead toconsiderable cost savings.

In addition, a significant expense associated with a dual acid etchedpart is the rate of rejects due to acid leaching out to surfaces whichdo not need to be etched. In the case of the composite implant 101 aaccording to certain embodiments of the present invention, the criteriafor acceptance of such a part will be lower because the majority of thesurfaces are covered with PEEK via injection molding after the dual acidetching process step. This feature can yield significantmanufacturing-related cost savings.

Although illustrated and described above with reference to certainspecific embodiments and examples, the present invention is neverthelessnot intended to be limited to the details shown. Rather, variousmodifications may be made in the details within the scope and range ofequivalents of the claims and without departing from the spirit of theinvention. It is expressly intended, for example, that all rangesbroadly recited in this document include within their scope all narrowerranges which fall within the broader ranges. In addition, features ofone embodiment may be incorporated into another embodiment.

We claim: 1-20. (canceled)
 21. An interbody spinal implant, comprising abody that is generally oval-shaped in transverse cross section, andcomprises: a top surface, a bottom surface, a posterior portion, ananterior portion comprising a solid rear wall extending substantiallythe entire width and height of the body, a substantially hollow center,and a single vertical aperture extending from the top surface to thebottom surface, having maximum width at its center, and defining atransverse rim on the top surface and on the bottom surface, saidtransverse rim having a blunt and radiused portion along the top of theanterior portion, wherein the portion of the transverse rim that is notblunt and radiused has a roughened surface topography, wherein the bluntand radiused portion does not include any roughened surface topography,and wherein the body has a sharp edge at the junction of the posteriorportion and the top surface and at the junction of the posterior portionand the bottom surface.
 22. The interbody spinal implant of claim 21,wherein the body comprises metal.
 23. The interbody spinal implant ofclaim 21, wherein the body comprises a non-metal polymer.
 24. Theinterbody spinal implant of claim 23, wherein the non-metal polymer isselected from the group consisting of polyetherether-ketone, hedrocel,and ultra-high molecular weight polyethylene.
 25. The interbody spinalimplant of claim 21, wherein the body comprises a composite of metal anda non-metal polymer.
 26. The interbody spinal implant of claim 21,wherein the solid rear wall closes the anterior portion of the body. 27.The interbody spinal implant of claim 21, wherein the spinal implantcomprises a lordotic angle adapted to facilitate alignment of the spine.28. The interbody spinal implant of claim 21, further comprising bonegraft material disposed in the substantially hollow center.
 29. Theinterbody spinal implant of claim 28, wherein the bone graft material iscancellous autograft bone, allograft bone, demineralized bone matrix(DBM), porous synthetic bone graft substitute, bone morphogenic protein(BMP), or a combination thereof.
 30. The interbody spinal implant ofclaim 21, wherein the posterior portion comprises an opening forengaging a surgical tool.
 31. An interbody spinal implant, comprising abody that is generally box-shaped in transverse cross section, andcomprises: a top surface, a bottom surface, a posterior portion, ananterior portion comprising a solid rear wall extending substantiallythe entire width and height of the body, a substantially hollow center,and a single vertical aperture extending from the top surface to thebottom surface, having maximum width at its center, and defining atransverse rim on the top surface and on the bottom surface, saidtransverse rim having a blunt and radiused portion along the top of theanterior portion, wherein the portion of the transverse rim that is notblunt and radiused has a roughened surface topography, wherein the bluntand radiused portion does not include any roughened surface topography,and wherein the body has a sharp edge at the junction of the posteriorportion and the top surface and at the junction of the posterior portionand the bottom surface.
 32. The interbody spinal implant of claim 31,wherein the body comprises metal.
 33. The interbody spinal implant ofclaim 31, wherein the body comprises a non-metal polymer.
 34. Theinterbody spinal implant of claim 33, wherein the non-metal polymer isselected from the group consisting of polyetherether-ketone, hedrocel,and ultra-high molecular weight polyethylene.
 35. The interbody spinalimplant of claim 31, wherein the body comprises a composite of metal anda non-metal polymer.
 36. The interbody spinal implant of claim 31,wherein the solid rear wall closes the anterior portion of the body. 37.The interbody spinal implant of claim 31, wherein the spinal implantcomprises a lordotic angle adapted to facilitate alignment of the spine.38. The interbody spinal implant of claim 31, further comprising bonegraft material disposed in the substantially hollow center.
 39. Theinterbody spinal implant of claim 38, wherein the bone graft material iscancellous autograft bone, allograft bone, demineralized bone matrix(DBM), porous synthetic bone graft substitute, bone morphogenic protein(BMP), or a combination thereof.
 40. The interbody spinal implant ofclaim 31, wherein the posterior portion comprises an opening forengaging a surgical tool.
 41. The interbody spinal implant of claim 31,wherein the body comprises a D-shaped box in transverse cross-section,wherein the transverse rim comprises a curved anterior portion and asubstantially flat posterior portion, thereby forming the D-shaped box.